Electrical network



July 6,1926. 1,591,073

o. J. ZOBEL ELECTRICAL NETWORK Filed Dec. 15, 1922 2 Sheets-Sheet 1 30 L Mel-ago 1 1 055,00, g! ll] (1/ M 1 MW Emu/ 62) B) (4 25 '3 2o is 15 g 40 g c All VYYVY AlAAl INVEN 10 0, JZad 6 ATTORNEY Patented J uly 6, I926.

NI-TED STATES PATENT 'oFFicE;

o'rro .r. ZOBEL, on NEW Yonx, N. n, ASSIGNOR T0 AMERICAN TELEPHONE AND LE- GRAPH comra w, A CORPORATION 0E NEW yonx.

ELECTRICAL NETWORK. v

. Application filed December 15, 1922. Serial No. 607,186.

An object of my invention is to provide a new and improved network for electric currents that shall have certain desired propbecome apparent in the following specification and claims, taken with the accompanying drawings, in which I have disclosed a single specific embodiment of the invention by way of illustration and example. With the understanding that the invention is defined in the appended claims, I shall now proceed to describe the particula f example thereof shown in the drawings.

Figure 1 is a diagram exhibiting attenuation characteristics of a certain non-loaded open-wire line which itis desired closely to simulate in a network for testing purposes; .Fig. 2 is a diagram of a general ladder type network; Fig. 3 is a diagram of a special case of this type of network which was developed to conform to the requirement based on Fig. 1; Fig. 4 is a diagram showing "how any desired number of half-sections of this network may be employed; Fig. 5 is a dlagram showing how certain elements of a section of the network of Fig. 3 may be varied toadjust the'equivalent ength of the sec-' tion; Fig. 6 is a diagram showingattenuation as a function of frequency for several difierent equivalent section lengths as determined in accordance with Fig. 5; Fig. 7 is a diagram showing deviations of I attenuation from an ideal, exhibited as a function of equivalent length of section. for several different frequencies; and Fig. 8 1s a diagram showing characterlstic impedance sideration.

A certain non-loaded open-wire line extendin between Pittsburgh and Qhicago is d1- vided into sections for o ratmg purposes, as indicated in- Fig. 1. ese sections have attenuation frequency characteristlcs as shown in the lower par of. Big 1- I W111.

function of equivalent. length for two 4 extreme frequencies of the range under co'nbe .seen that for any one of the sections, uniform increments of frequency correspond to uniform increments of attenuation. In

other words, attenuation is a'linear function I of frequency. In all cases, the diagrams correspond with the general equation w +7) p where d is the attenuation in standard miles, Z'1s the length of the line, 7 is the frequency, and a and 0 are constants.

The characteristic impedance of these line sections is substantially a constant resistance,

as'is the case for any long non-loaded openwire' line. In this particular case the characteristic' impedance is approximately 600 ohms of resistance.

'It is desired to produce a network of lumped impedance elements adapted by adustment to represent any section of this line, both in respect to characteristic im-.

pedanc'e and in respect to attenuation over a range of from ,3 to 30 kilocycles.

Such a network'may be obtained by applying the principles -of my present invention. First I assume that the desired network will. be of the general ladder type shown in FigLz-2, which comprises a succession of series impedances'z and alternating shunt impedancestz connected as shown 1n the diagram. I impose the condition that where k is a real constant. As is common in the treatment of such cases of recurrent networks, it will be assumed initially that the structure extendsindefinitely to the right,

as su gested by the dotted lines.v With theswitches as shown in Fig. 2, the network has -m1d-series termination at the input end at the left,that is, it begins with a series element of impedance value 2 easily shown that the mid-series characterlstic impedance is K. k /1 (a) If the switches are changed from the posi--' tions shown in Fig. 2, it will be seen that the network begins with a shunt element of half the normal admittance value, that is the network begins with a shunt element of impedtime alue g2... This is a midhun ter mination and, it is easily shown that the midshunt characteristic impedance 1s Consideration of equations (3) and (4) shows that if z, is kept small in absolute value relatively to k, it follows that K, and K will each be vectors of small equal but opposite angle and of absolute value nearly equal to the real quantity is. For the pres ent let it be assumed that 102600, which is the number of ohms in the characteristic resistance of the line to be simulated. It re-' -mains to find an appropriate structure for 2,, such that it will have a comparatively small absolute value of impedance over the desired frequency range of 3 to kilocycles, and such thata section of the network will havean attenuation which will be an increasing linear function of frequency.

From preliminary trials it appears that a section having an attenuation about the same as 40 miles, of the open-wire line will be about thelimiting equivalent length that can be obtained without too great an impedance change due to the maximumvalue of t encountered in -the contemplated frequency range. From Fig. 1 and equation (1) 1t is derived that the ideal attenuation for a 40-mile section increases linearly from 0.270

have the following values:

L ,=6.37ld.10' :727? R =O.36k L,,=10.37k.10-

In accordance with equation (2 the foregoing values determine, the values of the elemegts of 2,, which are as follows: f 6.37 21 7 3%? R,,s=2.779-7c w' T 1 In obtaining the foregoing values for the elements of 2,, I need haveno concern about quencles,

the effect on K, or K -due to varyin' 2 provided I keep. its absolute value smal as 'compared to, 70. Also, I have no concern about 2 for its valueis determined by 2 Therefore, I have only to seek for a proper combination of elements to represent 2 so I that I shall get a straight, upward-sloping line for the attenuation frequency characteristic.

It will be seen that in the foregoing tabu: lated values for the elements of z, and 3 R and R are expressed in terms of a constant T as yet undetermined. It wi-ll also be seen that in Fig. 3'these two resistance elements R and R are shown adjustable. It can be shown that if they. arevaried simultaneously in accordance with the curves shown in Fig.5, and without varying any. other elements of a, or 2 the equivalent length of the section,that is, its attenuatial change in the characteristic impedance.

Inasmuch as the characteristic impedance is approximately 600 ohms of resistance, the infinite network of Fig. 2, with; its details shown in Fig. 3, will behave the same at its input end, if it is terminated at the end tion, will be changed without any substanof any section by a resistance of 600, ohms.

Accordingly, such atermination is-shown in Fig. 4 with a system'of switches-by which any'desired number of half-sections may be employed between the input end at the leftand the terminal network at the right. Since the characteristic impedance is approximately the same at mid-series and midshunt as shown 'byequations (3) and (4), the

termination'of the network may be made at any mid-series ormid-shunt point and this enables'us to employ any desired number of half-sections, the attenuation in each halfsection being the same as for 20 miles of the open-wire line if the whole section corre-' sponds to 40 miles. With the design of Fig. 3 and the values for the elements thereof that have heretofore been written out in this specification, and with 10:600 ohms from 32 to 47 miles of the open-wire line of Fig. 1. In Fig. 6 the attenuation is plotted against frequency for each of three equivalent lengths at the extremes and at an intermediate of the range oflengths considered. It will be seen that the agreement with the-ideal is closer for a length of 40.20

' miles than for the lesser length of 32.20

miles'or the greater length of 46.23 miles. The same data are exhibited differently in Fig. 7 where the deviation of the attenuation from-the ideal is plottedagainst equivalent length for each of three representative freand' here agaimthe. 40 mile length shows up the most 6 and 7 1t appears that the best attenuation 1 and'with R and R given corresponding values within the range of Fig. 5, a single section has attenuation corresponding to avorably. From Figs.

frequency characteristic is obtained with a 40 mile section, and, accordingly, that is as sumed as the optimum, but lengths a little greater or a little less may be had by adjustment of the resistances R and R in accordance with Fig. 5. ,Also, a 20 inile length may be had by taking a half-section, for the reasons that have already been explained, and the same percentage variation from 20 miles may be had by adjustment of R and R- in accordance with Fig. 5. Applying these principles, it is readily shown that thesuccessive sections of Fig. 1 from Chicago to Pittsburgh may be represented respectivel by:

(1 3 1 sections each 39.1 miles long; (2) 4 sections each 38.5jmiles long; (3) 2 sections each- 39.5 miles long; (4;) 4 sections each 42.0 miles long. It will be seen from Fig. 8 that for a IO-mile equivalent section, when 10:600, K

560 to about 650, giving likewise'about 8% deviation.

While I have explained my network in relation to a special case, it will readily be seen that it offers a solution of the problem of gettinga network of approximately constant resistance characteristic impedance and with a desired relation between attenuaation and frequency.

I claim: J 1. A network of the type having successive series impedances 2 and in alternation therewith successive shunt impedances 2,,

in which each of said impedancesvcomprehends resistance elements, and subject to the condition that the product of 2 and 2 issubstantially a real constant for various frequencies.

2. network of ladder type with the product constant for its series impedance and its shunt impedance and with resistance elements in those impedances.

, 3. A ladder type network of substantially constant resistance characteristic impedance and having resistances in its impedance elements and having a predetermined attenuation frequency characteristic. j

4. A network to simulate a non-loaded upon wire line with successive series impedances and alternately disposed shunt impedances, each series impedance consisting of a series inductance and two branches in parallel with respect to said inductance, one such branch comprising a'resistance and another inductance and the other branch having an adjustable resistance, and each shunt impedance branchesin one of which there is a condenser and in theother of which there is an adjustable resistance, and in parallel with respect thereto anotherre'sistance and another condenser.

5. A network to simulate a non-loaded open wire line-with successive series impedances and alternately disposed shunt impedances,'each series impedance comprising reactance and resistance elements and each consisting of two parallel shunt impedance comprising reactan e and resistance elements, and the product of a series impedance value and a shunt impedance value'being equal to a real constant.

' 6. Anetwork of constant resistance characteristic impedance and with attenuation a linear functlon of frequency, said network COIHPIlSlaIif an adjustable series resistance and an justable shunt resistance to vary the degree of attenuation.

7. A sectional network of constant resistance characteristic impedance and with attenuation per section the same function of fre uency as for acertain length of nonloa ed open-wire line, and means to adjust th'e equivalent lengthQof a section within a narrow range and means to'combine sections. in sequence.

- In testimony. whereof, ,I have signed my name to this specification this 14th day of December, 1922.

013170 'J. ZOBEL. 

